1. Field of the Invention
The present invention relates to a method and to a circuit for transmitting data in a communication system, and more specifically, to more reliable and efficient methods and circuits for selecting precoding matrix for the open-loop structures.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a popular wireless communication technology used to multiplex data in the frequency. The total bandwidth in an OFDM system is divided into narrowband frequency units called subcarriers. In frequency-selective multi-user scheduling, a contiguous set of subcarriers potentially experiencing upfade distortion is allocated for transmission to a user. In frequency-diversity transmission, however, the allocated subcarriers are preferably uniformly distributed over the whole spectrum.
In a wireless mobile system employing OFDM based access, the overall system performance and efficiency can be improved by using, in addition to time-domain scheduling, frequency-selective multi-user scheduling. In a time-varying frequency-selective mobile wireless channel, it is also possible to improve the reliability of the channel by spreading and/or coding the information over the subcarriers.
A multiple antenna communication system, which is often referred to as a multiple input multiple output (MIMO) system, is widely used in combination with OFDM technology, in a wireless communication system to improve system performance. MIMO schemes use multiple transmitting antennas and multiple receiving antennas to improve the capacity and reliability of a wireless communication channel.
A popular MIMO scheme is MIMO precoding. With precoding, the data streams to be transmitted are preceded, i.e., pre-multiplied by a precoding matrix, before being passed on to the multiple transmitting antennas in a transmitter. In a pre-coded MIMO system, inverse operations are performed at the receiver to recover the transmitted symbols. The received symbols are multiplied with the inverse precoding matrices.
Recent efforts of the precoding approach were applies to both transmit diversity and MIMO spatial multiplexing. A composite precoder is constructed based on a unitary precoder such as Fourier matrix precoder multiplied with another unitary precoder representing a transmit diversity scheme such as Cyclic Delay Diversity (CDD). It should be noted that the principles of the current invention also applies to the cases of non-unitary precoding or unitary precoders other than Fourier matrix precoder. Matrix D is introduced as a symbol for a CDD precoding matrix and Matrix P is introduced as a symbol for a Discrete Fourier transform (DFT) matrix, then the combined matrix C=DP becomes column permutation on alternative subcarriers. Affords has been made to improve precoding methods in both of open loop structures and closed loop structures in following 3rd Generation Partnership Project (3GPP TM) documents:    [1]. 3GPP RAN1 contribution R1-072461, “High Delay CDD in Rank Adapted Spatial Multiplexing Mode for LTE DL”, May 2007, Kobe, Japan;    [2]. 3GPP RAN1 contribution R1-072019 “CDD precoding for 4 Tx antennas”, May 2007, Kobe, Japan;    [3]. 3GPP RAN1 contribution R1-072633 “Updated TS36.211 v1.1.0”, May 2007, Kobe, Japan;    [4]. 3GPP 36211-110: “3GPP TS 36.211 v1.1.0 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Channels and Modulation Release 8”, March 2007.
In an alternative precoding CDD structure, a large-delay CDD is applied in conjunction with the precoding matric, if a feedback of Precoding Matrix Indication (PMI) is available. For Large-delay CDD with PMI feedback, the codebook shall be selected from the Single User MIMO (SU-MIMO) codebook or a subset thereof. For large-delay CDD, precoding for spatial multiplexing shall be done according to the following equation:y(k)=W(k)QD(k)Ps(k),  (1)
where the precoding matrix W(k) is the channel-dependent default precoding (sub)matrix which is selected from a codebook of size Nt×p. Note that k is the subcarrier index, Nt is the number of antenna ports in transmitter and p is the transmission rank. The matrices P, and D(k) are of size p×p, while W(k) is Nt×p. The choice of Q can be of several different forms. Q=I where I is p×p identity matrix (in this case Q can be removed); or Q=P−1 which is the inverse of P.
In the contemporary methods for obtaining W(k), it is assumed that the choice of W(k) is chosen according the PMI, which is obtained from uplink feedback. Once a PMI is obtained for a subband, the same choice of W(k) is applied throughout this subband. That is, W(k) stays the same within the same subband. However, in the high speed scenarios the PMI feedback is not reliable and the PMI in the feedback cannot be used. The high speed system may be defined as an open-loop system. It is therefore not clear how the precoder W(k) should be selected in an open-loop system. Furthermore, the prior methods have no solution for the cases where no PMI is available for the less than full rank case.